The present invention relates to a new family of catalysts for the partial oxidation (CPO) of hydrocarbons, Steam Methane Reforming (SMR), Steam Naphtha Reforming or dry reforming (DR). The catalytic production of synthesis gas is operated under drastic conditions, mainly temperature conditions, which are very often detrimental for the stability of the catalyst. The catalytic partial oxidation of hydrocarbons (CPO) is for example a strong exothermic reaction and the first part of the catalytic bed can reach a temperature up to 1000° C. On the contrary, the steam methane reforming (SMR) and the dry reforming (DR) being endothermic reactions, it is thus necessary to heat up the catalytic bed close to 1000° C. in order to achieve the maximal conversion of the hydrocarbons species and the heat transfer to the catalytic bed must be done very quickly, generally less than one second and preferably less than 0.1 second, to decrease its contact time with the reacting species, in order to limit the carbon formation.
These reactions thus need use very stable catalysts in terms of metal sintering, support chemical, mechanical resistance and particle dispersion.
Furthermore, the activity of the catalyst activity may affect the thermal profile of the reactor: in this respect, it can be stressed that, the catalytic partial oxidation of hydrocarbons (CPO) must be carefully managed to avoid the formation of hot spots in the reactor, which can light on the homogeneous reaction; the endothermic reactions need a lot of energy which is brought by heating device heating the wall of the reactor. The homogeneity of the reaction could thus be facilitated even improved if the catalytic material leads to a good thermal transfer.
Moreover, active catalysts can improve the efficiency of these systems, if they are deposited on a support having a high thermal conductivity. Special and high-temperature resistant alloys typically offer this property
The catalytic bed must also induce to the smallest pressure drop, a pressure drop being detrimental to the reaction, while having the lowest bulk density mainly for economical reasons. The best compromise in terms of morphology and of geometry would probably be the use of support of a specified metallic foam kind.
Another key point concerns the method of deposition of the active phase on the support. As the specific surface area of a metallic support is null, a direct standard method of deposition such as the impregnation method, is unsuitable.
Several publications disclose the way of manufacturing supported metal catalysts, which are suitable for the oxidative reforming of hydrocarbons.
Japanese patent 5,186,203 discloses a catalytic element for SMR consisting of nickel fine particles which are impregnated on a porous alumina layer itself linked to the inner surface of a metallic reforming tubes.
Ismagilov et al. (Studies in surface Science and Catalysis, Elsevier, Amsterdam, 2000, Vol. 130 C, 2759) disclosed a catalytic heat-exchanging (HEX) tubular reactor to combining both an exothermic combustion and an endothermic steam reforming of methane, which comprises in the combustion part, a perovskite or Platinum supported catalyst on a Nickel-Chromium foam (Ni—Cr foam) material and in the steam reforming part, a Nickel-containing foam catalyst.
U.S. Pat. No. 6,630,078 B2 discloses the use of a metallic material as a catalytically support for the SMR reaction performed at low contact time value.
US Patent Application US 2003/0,185,750 A1 and International Application WO 02/066,371 disclose, that the active phase (Ni, Rh, Pt, Ru . . . ) is deposited on a ceramic spinel (Mg—Al203, La—Al203, Ce—Al203) even supported on a support standard alumina or on special metallic foam. Such a catalyst is used in SMR reactions, which the contact time required for, is shorter than 1 second.
US Patent Application US 2004/0,157,939 A1 discloses a catalytically active metal which is deposited on a silicon carbide support and used in the catalytic partial oxidation of methane to synthesis gas.
International Application WO 2004/087,312 discloses a simple and effective method for coating the surface of a metallic carrier material with a Ni catalyst, that can be used even if the carrier material has a complicated surface geometry.
US Patent Application US 2005/0,084,441 A1 discloses the preparation of C-nanochips, which are suitable and highly conductive supports for metals or metal oxides from the metals from Groups VIII, IB and IIIB of Periodic Table of Elements, and which may be used in catalytic reactions such as oxidation, hydrogenation, reforming or steam reforming.
Japanese Patent 5,007,298 discloses a porous metal catalyst usable for the steam reforming of hydrocarbons, which can be made by forming a catalyst layer comprising a porous ceramic layer carrying a catalyst powder, which is carried on the surface of a porous metal catalyst carrier having a 3-dimensional reticular structure. A composite ceramic powder formed by coating the ceramic powder with the catalyst powder is then fixed and carried on the surface of the skeleton of the porous metal carrier having the 3-dimensional reticular structure.
The deposition of the active phase is carried out by classical methods such as dip coating, successive wash-coats, organometallic vapor deposition, plasma deposition, or chemical vapor deposition, However, the metallic support must previously be coated by a ceramic layer such as MgAl2O4, Al2O3, Al2O3+CeO2, SiC Ca—Al203, or La—Al203, but the coating must have the lowest thickness as possible, in order to create a minimal surface area which is necessary to fix the active phase. The interest of this ceramic coating on the alloy foam is not only to ensure the catalytic active phase dispersion but also to protect the alloy along the time, under industrial conditions from corrosion, metal dusting or oxidation effects. The quality of this coating directly impacts the performances of the material in terms of stability or of catalytic activity and the catalytically active phase is not in direct contact with the metallic support. The known coating techniques cited above are however not easy to be worked on.
The starting point of the work of the inventors are two papers which reported the bulk synthesis of Hydrotalcite compounds containing Co or Ni as bivalent cations and Al as trivalent cations by cathodic reduction of nitrates (L. Indira and P. V. Kamath, J. Mater. Chem. 4 (1994) 1487; Dixit and P. V. Kamath, J, Power Sources 56 (1995) 97]. On this basis, an extensive study has been performed to find the best experimental conditions to modify different electrodes by one-step electro-synthesis of Ni/Al Hydrotalcite (E. Scavetta, B. Ballarin, M. Giorgetti, I. Carpani, F. Cogo and D. Tonelli, J. New Mater. Electrochem. Systems 7 (2004-43), evidencing the key role of time and potential to control the film thickness. FIG. 1 discloses a plot of the film weight as a function of the deposition potential and time (for a Pt electrode). FIG. 2 is an optical microscope photograph of the HT film obtained on a Pt electrode at E=−0.9V and t=10 s which is a very clean and stable film.
FIG. 3A) discloses a plot of the electrical intensity versus time for two different Pt electrodes during the Ni/Al—NO3 Hydrotalcite compound electrosynthesis at E=−0.9 V.
FIG. 3B) discloses the cyclic voltamogramms (20th cycle) in a decimolar aqueous soda solution (0.1 M NaOH) for two electrodes in Platinum modified with Ni/Al—NO3 Hydrotalcite compound which was electro-synthesized in a potentiostatical way at E=−0.9 V for 10 s; potential scan rate=50 mVs−1. Both figures evidence the high reproductibility of the technique.
These modified electrodes have been already applied in a flow system for the amperometric determination of sugars and alcohols [B. Ballarin, M. Berrettoni, I. Carpani, E. Scavetta and, D. Tonelli, Anal. Chim. Acta 538 (2005) 219]. Mixed oxides bulk electrosynthesis has also been reported, in particular ferroelectric lead zirconate titanate (PZT) [Zhitomirsky, A. Kohn and L. Gal-Or, Mater. Letters 25 (1995) 223], rare-earth chromates Ln2Cr3O12×7H2O (Ln=La, Pr, Nd) [G. H. Annal Therese and P. Vishnu Kamath, Mater. Res. Bulletin 33 (1998) 1] or Ba5Ru3Na2O14 10H-perovskite related structure [E. Quarez and O. Mentré, Solid State Science 5 (2003) 1105.].
The aim of the present invention is thus to propose a new approach which dramatically reduces the number of process steps, by proposing a direct “active dense ceramic coating” on the alloy foam.